Downhole reciprocating and rotary pumps are positioned and actuated in a wellbore by a rod string extending from surface. The rod string is typically either one continuous member or a plurality of sucker rods, connected end-to-end through standard threaded couplings.
It is known that downhole pumps may become lodged or stuck in a wellbore, often by sand deposited and packed around the pump, either at the downhole pumping location or as the pump is being tripped out of the wellbore. Conventionally, the rod string is removed from the pump by applying a pulling force on the rod string to sever the rod string from the pump.
A shear coupling assembly is typically used to connect between the pump and a downhole end of the rod string. The shear coupling primarily functions to provide a means for separating the rod string from a stuck pump so as to release and remove the rod string from the wellbore and permit specialized equipment to be inserted into the well annulus to free the pump. Use of the shear coupling at the interface between the rod string and the pump provides a specified location at which the pump and rod string are separated and the shear coupling can be constructed to actuate under a desired design load which is highly predictable. Without the shear coupling, the rod string would sever at a location along the rod string that is unknown and largely unpredictable and which can be problematic for retrieval of the pump.
It is known to use a shear coupling comprising transversely extending shear pins for joining male and female coupling members between the pump and the rod string. The shear pins are known to be prone to premature fatigue which arises from cyclic compressive stress induced in the shear pins in a reciprocating pump if the rod string “taps down” at the base of each reciprocating stroke. Further, in either a reciprocating or rotary pump, as the shear pins break, fragments fall downhole into the pump, resulting in further problems in freeing the pump.
In an effort to solve the problems associated with previous shear coupling designs, shear couplings, such as taught in Canadian Patent 1298715 to Mann et al, are known to utilize a threaded connection between a pin coupling member, having an externally threaded head, and an internally threaded box coupling member. Either of the pin coupling member or the internally threaded box coupling member is connected to the pump and the other is connected to the downhole end of the rod string. The threaded head of the pin coupling member threadedly engages the internal axial bore of the internally threaded coupling member for operatively connecting therebetween. The pin coupling member further comprises a shear neck of reduced diameter between the head and a body of the pin coupling member which is designed to shear under design load to free the pump from the rod string.
During assembly, a pretension is typically applied to the shear neck of the pin coupling member during threaded connection to the box coupling. The box coupling seats on a shoulder of the pin coupling so as to maintain the shear neck in tension during normal operation of the pump for preventing premature fatigue of the shear neck. Shear couplings of this design are particularly suited for use in reciprocating pumps but are not useful for rotary pumps as the shear element would take virtually 100% of the torsional load.
Canadian Patent 2,425,091 to Bostik teaches a one-piece shear coupling for use with both rotary and reciprocating pumps. The shear coupling comprises a cylindrical body which is adapted at one end for connection to a rod string and at the other end to the pump. A weakness, such as a groove, is formed in the body therebetween so as to provide a stress-concentration point for shearing upon being subjected to a predetermined amount of stress. At least the groove must be treated with an anti-corrosive material as the entirety of the cylindrical body is exposed to potentially corrosive wellbore fluids. One of skill in the art would understand that if designed for tensile loading, the addition of torsional loading as a result of rotary operation would result in premature failure. Further, in the case of axial operations, the groove acts as a stress concentration when subjected to bending forces, such as in a deviated wellbore.
There is continued interest in the industry for shear coupling assemblies which provide reliable shearing under design conditions while avoiding the problems associated with parts which may fall into the wellbore or pump after shearing, avoid the effects of corrosive wellbore fluids, are relatively simple and inexpensive to manufacture, and can be used in both reciprocating and rotary pump applications.